Embodiments of the present invention relate to integrated circuits and the processing for the manufacture of semiconductor devices. More particularly, embodiments of the invention provide a method and device with multiple gate oxide thicknesses. Merely by way of example, the invention has been applied to CMOS image sensing. But it would be recognized that the invention has a much broader range of applicability.
Integrated circuits or “ICs” have evolved from a handful of interconnected devices fabricated on a single chip of silicon to millions of devices. Current ICs provide performance and complexity far beyond what was originally imagined. One such type of IC is a CMOS imaging system. The CMOS imaging system can be fabricated on standard silicon production lines and is therefore inexpensive to make. Additionally, the CMOS image sensor consumes low power and is especially suitable for portable applications.
Specifically, a CMOS image sensor converts a light signal into an electrical signal, whose intensity is related to the light intensity. FIG. 1 is a simplified diagram of a conventional CMOS image sensor. The CMOS image sensor 100 corresponds to one pixel and includes a reset transistor 110, a photodiode 120, a source follower 130, a selecting transistor 140, and a bias resistor 150. The photodiode 120 receives a light signal and generates a photocurrent from a node 160 to a node 170. Additionally, a leakage current also flows through the photodiode 120 in the same direction. One source of the leakage current is the source region of the reset transistor 110, which is connected to the photodiode 120.
FIG. 2 is a simplified conventional diagram of the reset transistor 110 and the photodiode 120. The photodiode 120 includes an active region 210, and the reset transistor 110 includes a source region 220, a drain region 230, and a gate region 240. The source region 220 forms a junction with the substrate or a well in the substrate, and the junction usually experiences certain leakage. The leakage is usually passed to the active region 210 and contributes to the leakage current of the photodiode 120. A large leakage current adversely affects the performance of the CMOS image sensor.
FIG. 3(a) is another simplified conventional diagram for the reset transistor 110 and the photodiode 120. The photodiode 120 includes a diode diffusion region 310 formed under a gate oxide region 320. The reset transistor 110 includes a source region 330, a drain region 340, and a gate region 350. Since dark current is a main issue associated with a CMOS image sensor, low leakage current of the CMOS image sensor is needed. The current leakage from the reset transistor 110 source region 330 to the gate region 350 due to similar phenomenon as gate induced drain leakage (GIDL) effect is one of the main sources for this leakage current. Reduction of the electric field in this area is needed to reduce GIDL effect.
FIG. 3(b) is yet another simplified convention diagram for the reset transistor 110 and the photodiode 120. The photodiode 120 includes a diode diffusion region 355 formed under a field oxide region 360. The reset transistor 110 includes a source region 370, a drain region 380, and a gate region 390. The source region 370 is connected to the diode diffusion region 355 and formed with a deep source implantation penetrating the gate region 390. The gate region 390 is not aligned with the source region, so the reliability of the CMOS image sensor usually deteriorates. Any misalignment would affect the Cgs (CMOS gate-to-source capacitance) uniformity, and in turn the output signal reset level uniformity, throughout the wafer, since Cgs has impact on the reset output signal level.
From the above, it is seen that an improved technique for CMOS image sensor is desired.